Food web biomanipulations provide the means to partly control water quality problems (i.e. algal blooms) in lakes by altering internal ecological mechanisms. However biomanipulation mechanisms are complex and the results are often confounded through the interactions of various stressors. This study used paleolimnological approaches using the fossil remains of diatoms, Cladocera and Chaoborus indicators to explore the effects of past fish manipulations of two oligotrophic lakes (Mouse and Ranger lakes) located in south-central Ontario. A major focus was the temporal period associated with the most recent Dorset Food Web Piscivore Manipulation Project (1991-1997). The diatom assemblages recorded only subtle changes over time, with the most pronounced variation in the past decade, likely responding to recent warming and bottom-up processes. Following the smallmouth (Micropterus dolomieu) and largemouth (Micropterus salmoides) bass addition to Mouse Lake, the zooplankton and Chaoborus fossil record revealed that a reduction in planktivorous fish led to expected shifts in community size structure, but with changes apparently delayed by ~5-10 years. Primary production, as inferred from VRS chlorophyll-a concentrations, had been relatively stable for over ~200 years but increased significantly following the piscivore addition. A shift to larger, more efficient herbivorous zooplankton (i.e. Daphnia) may have been expected to suppress phytoplankton as they are able to filter food particles at a much faster rate and graze a wider size-range of algae compared to the less effective grazer, Bosmina. Additionally, changes to the lake system from climate warming may have uncoupled the trophic interactions, producing a mismatch in the timing of favourable environmental conditions in an algal-herbivore interaction. In addition, these oligotrophic lakes support algal communities often classed as being ungrazable, which would also substantially reduce grazer-mediated responses in the phytoplankton. Ranger Lake did not respond to reductions to piscivores as expected in terms of zooplankton and Chaoborus size structure. Larger-bodied Cladocera (i.e. Daphnia, Holopedium gibberum and Polyphemus pediculus) increased in abundance while small Bosmina species decreased markedly in the uppermost sediments. While some results of this study corroborated the expected effects of biomanipulation theory (i.e. species-specific shifts in size), the inconsistencies encountered highlight that other drivers (i.e. climate) are likely to
mediate the effects of biomanipulations, particularly in temperate lake systems experiencing multiple stressors.